In thermally-assisted magnetic recording, also called heat-assisted magnetic recording, information bits are recorded on a data storage medium at elevated temperatures, and the data bit dimensions can be determined by the dimensions of the heated area in the storage medium or the dimensions of an area of the storage medium that is subjected to a magnetic field. In one approach, a portion of the medium is heated to reduce the magnetic coercivity, and data is then written to the reduced coercivity region.
The data is stored and retrieved by a transducer, or “head,” that is positioned over a desired track under control of a closed-loop servo system based on position information, or “servo data,” which is stored within dedicated servo fields located on the storage medium. The servo fields can be interleaved with data sectors on the disc surface or can be located on a separate disc surface that is dedicated to storing servo information. Typically, servo-track writing is done with a head significantly wider than the recording head with subsequent tracks overlapping to provide complete servo information over the entire disk from the inner diameter to the outer diameter. The critical importance for servo-track writing is that the tracks have sharp edges.
The use of overlapping, or shingled, tracks has also been proposed to increase data storage density. For shingle write recording, a wide initial track is written, and only an edge of that track remains after the track adjacent to it is written.
Two types of optical structures have been proposed for use in heat-assisted magnetic recording (“HAMR”) recording heads. A Solid Immersion Mirror-only (“SIM-only”) structure can be used to produce a diffraction-limited spot of optical energy. A Near Field Transducer (“NFT”) structure includes a Near Field Transducer that can concentrate optical energy in a very small spot. Widening the track of a SIM-only head while preserving the same edge sharpness is not trivial, because reducing the focusing of the optical spot would also reduce the optical gradient at the edge of the track. Widening the track of an NFT head is not trivial because the power delivered by the NFT to the media scales linearly with the width of the track. Doubling the track width would require double the power, possibly beyond the capabilities of the light delivery system.
There is a need for a HAMR recording head that can be used for writing servo tracks or overlapping tracks.